Understanding the role hierarchy
Every user record in Salesforce has the option to be added to a role. That role is part of an overall hierarchy. The most common use case is when someone higher in the role hierarchy inherits the permissions to objects of users that are below them. For example, the sales manager role inherits the permissions of someone in the sales rep role as the manager comes above the sales rep in the hierarchy. There are also instances such as Sales Ops where the user may not be higher in the organizational hierarchy, but sits higher on the role hierarchy due to the need to have visibility in all levels of the business.
Business use case
As the Salesforce admin for XYZ Widgets, you need to limit the visibility of accounts to account owners and their managers. The previous step of setting up the org-wide default settings for accounts as private assures you that only the account owners can see the accounts they own. Setting up a role hierarchy will take care...